In recent years, a small size, a light weight, a high-speed, high image quality, and high reliability have been severely pursued for electrophotographic image forming apparatuses including a copying machine and a laser beam printer to meet the requirements of space savings, energy savings, and the like. Therefore, the image forming apparatuses have been structured with simple components in various parts. As a result, the performance demanded for toner has become more sophisticated so that a more excellent image forming apparatus cannot be established unless an improvement in toner performance is achieved. In addition, with the advent of recent various needs, the demand for full-color image output has been surging. In view of the above circumstances, additional increases in image quality, resolution, and the like have been demanded.
An improvement in color reproducibility and transparency of an OHP image are important for a color toner to be mounted on a typical full-color copying machine. Therefore, a sharp-melt and low-molecular-weight polyester resin or the like is used as a binder resin and the color toners of the respective colors are designed to be sufficiently mixed in a fixing step. However, such a resin having sharp-melt property poses a problem in that a hot offset phenomenon in which a molten toner adheres to a fixing roller or the like occurs owing to weak self-cohesive force of the resin. Silicone oil or the like has been conventionally uniformly applied to the fixing roller for the purpose of preventing the hot offset phenomenon. However, an image obtained with this arrangement has excessive silicone oil or the like adhering to the surface of the image. Therefore, the image is not preferable because a user has a feeling of discomfort particularly when using the image in an OHP image.
On the other hand, a black toner for a monochrome copying machine and a monochrome printer, which is widely used in the market, often contains a wax for preventing offset to eliminate the need for applying silicone oil to a fixing roller. Attempts have been recently made to allow a toner for full-color to contain a wax. However, as described above, a toner for full-color has poor compatibility with a wax because the toner is generally composed of a polyester resin. As a result, the wax is insufficiently dispersed so that the fixing performance becomes insufficient. In addition, various problems associated with the developability, durability, storage stability, and the like of the toner occur.
Various propositions have been made to such a problem of insufficient dispersion of a wax into a polyester resin.
For example, JP 11-352720 A proposes a toner in which the dispersibility of a wax into a binder resin has been improved by using a hybrid resin synthesized from a mixture composed of a vinyl-based monomer for forming a vinyl-based copolymer, acid and alcohol components for forming a polyester resin, and the wax.
In addition, JP 2003-076066 A proposes a toner containing at least: a wax dispersant obtained by grafting a copolymer, which consists of styrene, a nitrogen-containing vinyl monomer, and a (meth) acrylic acid-based monomer, into a polyolefin; a hydrocarbon-based wax; and a hybrid resin, the toner having good dispersibility of the wax and satisfying a high gloss excellent in color mixability and permeability.
Furthermore, JP 2003-076056 A proposes a toner having a main peak in the molecular weight region of 5,000 to 70,000 and Mw/Mn of 100 or more. In the toner, the formation of a domain of 0.01 to 5 μm by primary dispersed particles containing a wax each having a dispersion particle size in the range of 0.001 to 4 μm can be observed by cross-section observation of the toner with a focused ion beam (FIB). In addition, JP 2003-076056 A proposes a toner having an average circularity in the range of 0.92 to 0.96 and a precipitation starting point at a methanol hydrophobing in the range of 35 to 60 vol %. In the toner, primary dispersed particles containing a wax each having a dispersion particle size in the range of 0.005 to 4 μm form a domain of 0.01 to 5 μm.
Furthermore, JP 3225889 B proposes a toner which is allowed to contain 0.1 to 40 mass % of wax and to have a presence ratio of wax exposed to the toner surface in the range of 1 to 10 mass % by mixing a solution of a polyester resin dissolved in a solvent with slurry of a fine-particle-state wax and pigment slurry, granulating the mixture in water, and then distilling off the solvent at room temperature. In the toner, the shape of the wax is a flaky shape and the number average dispersion size of the wax is in the range of 0.1 to 2 μm.
However, it still cannot be said that those toners with improved wax dispersibility have fully optimized their wax dispersibility. Therefore, there has been demanded a toner in which the fixing performance (such as low-temperature fixability or hot offset resistance) has been further improved by making a wax finer and uniformer, in other words, by dispersing at least part of a wax uniformly at a molecular level into a binder resin.